In process automation technology, sensors are frequently applied, to serve for registering values of measured variables. As a rule, the sensors are connected via a measurement transmitter with a control system, to which the measurement data is forwarded. Communication between the measurement transmitters and the control system is accomplished according to one of the standards usual in process automation technology, such as e.g. HART data transmission or a field bus system (Foundation fieldbus, Profibus, etc.). Besides these open transmission systems, also sometimes applied in the case of digital data transmission are proprietary protocols, this especially being true in the case of peer-to-peer combinations between sensor and measurement transmitter.
In the case of certain sensor types, the sensors are often constructed of two components, a sensor head and a sensor-head counterpart fitting the sensor head. The sensor-head counterpart provided at a measuring point is fixedly connected with the measurement transmitter, while the sensor head can be removed. This dividing in two is especially advantageous in the case of sensors that must be regularly subjected to maintenance.
For cleaning or calibrating, the sensor head can be simply removed at the measuring point, in order then to be transported to a laboratory, where maintenance is performed.
Such a two-part sensor is available from the applicant (Endress+Hauser Conducta) under the mark Memosens®. This product has been manufactured and sold for a number of years by the applicant.
Problematic in the case of such sensors is that, after the cleaning or calibration, the right sensor head must be re-installed onto the sensor-head counterpart at the right measuring point. Frequently, the user carries a box containing a set of several sensor heads, out of which the right sensor head for the measuring point of concern must be selected. For this, the sensor heads are labeled with the measuring point designation (TAG-Name), in order to facilitate, for the user, the association of sensor head with measuring point.
Due to the manual activity of the user, however, mix-ups are still possible. A mismatch can have serious consequences. Thus, a measuring head, which was installed at a cyanide measuring point, must, under no circumstances, be installed at a foods measuring point.
A wrongly applied measuring head can lead e.g. to contamination of the product being measured and, therewith, to serious degradation of following processing steps of the product.
Currently, applications are known, where sensor and measurement transmitter are located relatively far from one another. In the future, it is planned that sensors will be able to be connected directly with the control system, without the interposing of a measurement transmitter.
A wrongly applied sensor head can, in principle, be indicated on the measurement transmitter or on the control system. When sensor and measurement transmitter or control system are separated relatively widely spatially, however, such a display directly at the measuring point is not possible
To check in the case of each replacement, the user would have to travel relatively long paths. This kind of checking is, moreover, extremely time consuming in the case of sets of, perhaps, 20 sensor heads, since the user must return to the control system a number of times, in order to check whether a correct sensor head has been installed.
All previously known methods of checking are burdened with possible sources of error, so that absolutely reliable installation of the sensor heads at their correct measuring points is not assured.
In the future, replacement of measuring heads should be a routine activity, which need not necessarily be performed by technicians. In this way, still higher requirements are placed on effecting safe replacement.